Mount Kulen, Phnom Kulen in Khmer, is a rugged mountain plateau northeast of Siem Reap that makes a great day trip any time of year, however, I love to visit during the monsoon. Most visitors to Kulen see the River of a Thousand Lingas, the Reclining Buddha, and have a paddle in the waterfall and leave it at that. We like to be a little more adventurous and hop on a bike behind some local moto blokes for a day bouncing along the stone plateau, crossing overflowing creeks, and hacking our way through overgrown jungle tracks to see the ruins of hidden temple towers, remnants of the so-called 'lost city' of Mahendraparvata. Take a raincoat and hat and make sure you have a waterproof bag for your camera and valuables, as things can get very wet up there. While you can do this experience independently, it's easier to do it on a private guided tour, staying nearby overnight so you can get there soon after sunrise, your moto boys know where to go, and you can see some of the secret sights. I like Backyard Travel's Phnom Kulen tours best as they can customize them to suit your interests and time available, organizing anything from two days to four days, taking in other remote temples, such as Koh Ker and Preah Vihear. Note: the plateau hasn't been fully cleared of landmines, so don't venture off paths or tracks.